The most common and most likely answer to this crossword clue is the 3 plus 2 letter word SIT UP
I've seen this crossword clue in Financial Times
S
I
T
U
P
More clues leading to the result SIT UP
- 'Tis suggested, don’t go to bed
- Have a meal, imbibing drink – and don’t slouch!
- A raised place is said to prevent slouching
- Don’t slouch in places one enters heading north
- Exercise and beg for a bicycle
- Nameless Russian leader’s about to delay retirement
- Become suddenly interested in Italian overcome by drink
- It need not be literally done to become alert
- Pay attention in part of exercise
- Pay attention to strengthening exercise
Buddy explains!

The straight part
We know that "Don’t slouch" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case don’t slouch = sit up.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SIT UP mean?
SIT UP noun- An exercise in which the abdominal muscles are contracted, bringing the torso of a person lying down to a more upright position.